Furnace for burning wet fuels and method of operating same

ABSTRACT

Furnace for burning wet fuels on a grate having a blast of hot air forced through the grate and a supplemental burner to provide additional combustion heat. The blast of hot air is heated to a greater degree by the feeding relatively cool combustion air to the supplemental burner and using an air heater, which normally heats such combustion air, to preheat the air forming the undergrate blast.

United States Patent [72] Inventor Earle C. Miller Worcester, Mass.

[211 AppLNO. 831,124

[22] Filed June 6, 1969 [45] Patented Jan. 26, 1971 [731 Assignee Riley Stoker Corporation Worcester, Mass. a corporation of Massachusetts [54] FURNACE FOR BURNING WET FUELS AND METHOD OF OPERATING SAME 12 Claims, 1 Drawing Fig.

[52] U.S.Cl 110/8, 110/56 [51] 1nt.Cl F23gS/l2 [50] FieldofSearch 110/7,8, 15,56

{56] References Cited UNlTED STATES PATENTS 2,032,402 3/1936 Colby et al 110/15 2,148,447 2/1939 Dundas et al. 110/15 2,592,491 4/1952 Toepel t. 1 10/8 Primary Examiner-Kenneth W. Sprague At1orneyWard, McElhannon, Brooks & Fitzpatrick ABSTRACT: Furnace for burning wet fuels on a grate having a blast of hot air forced through the grate and a supplemental burner to provide additional combustion heat. The blast of hot air is heated to a greater degree by feeding relatively cool combustion air to the supplemental burner and using an air heater, which normally heats such combustion air, to preheat the air forming the undergrate blast.

FURNACE FOR BURNING WET FUELS AND METHOD OF OPERATING SAME This invention relates to an improved furnace for burning on a combustion grate, wet fuels, for example, bark, bagasse and similar wet refuse fuels, as well as to an improved method of operating such furnace.

In burning wet fuels on a grate, it is common practice to supply an undergrate blast of hot air which evaporates some of the moisture from the fuels and furnishes the oxygen required for combustion. It is also common practice to provide a supplemental burner or supplemental firing near the combustion zone of such wet fuels in order to produce a higher temperature at the combustion zone and thus facilitate the combustion of the wet fuels on the combustion grate. In the usual case, these practices are simultaneously realized by providing rela tively cool air, such as with a forced draft fan, and passing the relatively cool air through an air heater located in the path of the flue gases of the burning fuels and arranged to transfer heat from the flue gases to the relatively cool air to provide relatively hot air. The relatively hot air produced in the air heater is then simultaneously fed both to the supplemental burner or the like to control the combustion thereat, and to beneath the combustion grate where it is forced upwardly to evaporate some of the moisture from the fuels thereat and facilitate combustion of same.

In burning very wet fuels in accordance with the above practices, I have found the need of an undergrate blast of air at a temperature higher than can be normally developed in the described air heater which preheats the air being fed both to the supplemental burner and to the combustion grate. While such higher temperature may be obtained by using one or more additional air heaters, such alternative is very expensive and not commercially desirable.

In accordance with the present invention, there are provided an improved furnace of the class described and an improved method of operating same which permit preheating of the air being fed to the undergrate, to a degree not previously possible and without the use of an air heater which is supplemental to the primary air heater commonly found in prior furnaces of the class described.

Briefly, in accordance with the present invention, I have found that in bypassing to the supplemental burner, a portion of the relatively cool air usually fed to the primary heater of furnaces of the class described, the remaining cool air passing through the primary air heater and forming the undergrate blast is raised to the higher temperatures required in the combustion of very wet fuels, Thus, since the amount of air passing through the primary air heater per unit time is less than in prior furnaces of the class described, under substantial isobaric conditions existing in the air heater, the heat transferred from the flue gases to the smaller volume of air passing through the air heater raises the temperature of such air to a higher degree. Interestingly, the described bypassing of the primary air heater and feeding less than relatively hot air to the supplemental burner, has little or no effect on the operation of the supplemental burner since such air is primarily used to facilitate combustion.

Thus, the improved furnace may comprise a combustion grate located in the combustion zone of the furnace of carrying the wet fuel. As in prior furnaces of the class described, a supplemental heater is located near the combustion zone for providing supplemental heat thereat; and a forced air fan is provided to supply relatively cool air to the furnace system. Of course, supplementary firing may be used instead of an actual burner. In accordance with the present invention, means are provided for supplying a portion of the relatively cool air directly to the supplemental burner to control the combustion thereat, and means are provided for supplying the remaining portion of the relatively cool air to the primary air heater found in furnaces of the class described so as to provide the desired relatively hot air. Of course, means are also provided in the furnace for passing the thus produced relatively hot air upwardly through the combustion grate to evaporate some of the moisture from the wet fuel and facilitate combustion of same.

The exact temperature desired in the undergrate blast may be realized by providing means, such as an air baffle, for regulating the amount of relatively cool air directly supplied to the supplemental burner, and thus, in addition, regulating the amount of the remaining air fed through the air heater to the undergrate. Also, if conditions warrant, the relatively cool air fed to the supplemental burner may be warmed by providing means, such as a connecting duct, for mixing a portion of the relatively hot air at the output of the primary heater with the relatively cool air fed to the supplemental burner, and by regulating the exact amount ofthe relatively hot air mixed with the relatively cool air.

There has thus been outlined rather broadly the more important features of the invention in order that the detailed description thereof that follows may be better understood and in order that the present contribution to the art may be better appreciated. There are, of course, additional features of the invention that will be described hereinafter and which will form the subject of the claims appended hereto. Those skilled in the art will appreciate that the conception on which this disclosure is based may readily be utilized as the basis for the designing of other structures for carrying out the several purposes of the invention.

A specific embodiment of the invention has been chosen for purposes of illustration and description and is shown in the accompanying drawings wherein the single FIGURE shows in somewhat schematical form an improved furnace 10 constructed in accordance with the present invention for burning wet fuels, such as bark, bagasse, and similar wet refuse fuels,

The furnace 10 has a combustion zone or chamber 12 enclosed by the walls 14 defining the sides of the furnace 10, a foundation floor 16, and a roof wall 18 defining the furnace roof. The walls 14 and the floor 16 may be lined with ceramic brick 19 or the like. Of course, other structural features such as boiler tubes and the like may be incorporated in the furnace 10 but are not shown or described for brevity.

A combustion grate, generally shown at 20 and comprising spaced-apart grate bars 22, is located in the lower portion of the combustion zone I2 and is arranged to carry wet fuels, such as bark, bagasse and similar wet refuse fuels, A supplemental burner, generally shown at 24, is provided at one of the sidewalls l4 and near the grate 20 for providing supplemental heat at the combustion zone 12 during the combustion of the wet fuel on the grate 20. The supplemental burner 24 may be of any form which achieves the desired stated function, and preferably comprises a mixing and burning nozzle 26 connected to a suitable fuel supply, such as an oil reservoir (not shown), by the conduit 28 and to a source of combustion air by an air duct 30 which is connected to a source of air in a manner hereinafter described.

A funnel-shaped register 32 is provided beneath the combustion grate 20 and extends up from an air duct 34 projecting through the floor 16 of the furnace 10. The register 32 is arranged to receive hot air supplied through the air duct 34 in a manner hereinafter explained, and to channel an undergrate blast of hot air upwardly through both the grate 20 and the wet fuel carried by the latter to evaporate some of the moisture from the wet fuel and furnish oxygen for combustion of such fuel.

The combustion air supplied to the supplemental burner 24 and to the register 32 forming the undergrate blast, is provided by a forced air fan 36 coupled to a suitable source of relatively cool air (not shown). A portion of the cool air supplied by the forced air fan 36 is fed directly to the supplemental burner 24 through the air duct 30, and the remainder of such supplied air is fed through an air duct 38 to an air heater, shown generally at 40. The air heater 40 is arranged to heat to a high temperature the relatively cool air supplied thereto by the air duct 38, and coupled the thus heated through the air duct 34 to the register 32 so as to form the desired hot undergrate blast.

The air heater 40 is arranged to transfer heat from the flue gases of the furnace to the cool air supplied thereto by the air duct 38. To this end, the air heater 40 may comprise tubing, shown schematically at 42, connected between the flue 44 at the roof 18 of the furnace 10 by an air duct 46 and an exhaust (not shown) by an air duct 48. The air heater 40 also may comprise tubing, shown schematically at 50, connected in heat exchange relation with the tubing 42 and arranged both to heat the cool air supplied by the air duct 38 and to carry same to the air duct 34 connected to the register 32.

In order to add some warmth, as desired, to the air supplied to the supplemental burner 24, a connecting air duct 52 and a baffle 54 are provided between the air duct 34 which supplies the hot air to the register 32 and the air duct 30 which directly supplies the relatively cooler air to the supplemental burner 24. Also, a baffle 56 is provided upstream of the air duct 54 to control the amount of cool air supplied to the supplemental burner 24, and thus to control the quantity and temperature of the air heated in the air heater 40 and supplied to the register 32. Finally, cool water may be circulated through conduits formed in the grate bars as at 58 and from a suitable cooling source, such as the boiler tubes, if any, of the furnace in order to protect the grate structure.

ln operation, the relatively cool air supplied by the forced air fan 36 is divided between the air ducts 30 and 38 in a ratio determined by the position of the baffle 56. The relatively cool air supplied to the air duct 30 is directly coupled to the supplemental bumer 24 to control the combustion of the fuel supplied through the conduit 28. The supplemental burner thus provides combustion heat in the combustion zone 12. The remaining portion of the cool air supplied by the forced air fan 36 is coupled by the air duct 38 to the air heater 40 where it is heated by the heat in the flue gases of the furnace l0 and then coupled by the air duct 34 to the register 32 to provide a hot air blast beneath the grate 20 sufficient to evaporate some of the moisture from the wet fuel on the grate 20 and supply oxygen to facilitate combustion As stated above, the exact temperature of the hot undergrate blast may be adjusted by moving the baffle 56 to regulate the amount of air supplied to the air heater 40. Also, if conditions warrant, the other baffle 54 may be adjusted so that a small amount of the hot air passing through the duct 34 to the register 32 mixes with the cool air supplied to the supplemental burner 24.

Thus, it will be appreciated from the above that the furnace it) permits the heating of the air fed to the register 32 which forms the undergrate blast, to a degree not previously possible and without the use of any costly supplemental air heaters.

I claim:

I. A method of burning in a furnace wet fuel which is placed on a combustion grate located in the combustion zone of said furnace, said method comprising: providing supplemental heat at said combustion zone by operating a supplemental burner being fed with air which controls the combustion at said supplemental burner; providing relatively cool air and supplying same to an air heater to produce relatively hot air; passing at least a portion of said relatively hot air upwardly through said combustion grate to evaporate some of the moisture from said wet fuel and facilitate combustion of such fuel; and bypassing a portion of said relatively cool air supplied to said air heater directly to said supplemental burner to control combustion thereof whereby the unbypassed portion of said relatively cool air passing through said air heater is heated to a greater extent thereby providing air of a higher temperature to evaporate moisture from said wet fuel and facilitate combustion of same.

2. A method in accordance withclaim 1 wherein said air heater is an air heat exchanger and wherein said relatively hot air is produced by passing flue gas of said furnace through said air heater.

3. A method in accordance with claim 1 wherein a portion of said relatively hot air is also supplied to said supplemental burner.

4. A method in accordance with claim I and further comprising the step of cooling said combustion grate.

5. In a method of burmng wet fuel in a furnace having a supplemental burner to provide additional heat in the combustion zone thereof. wherein said wet fuel is positioned on a com bustion grate in said combustion zone while relatively cool gas is passed through an air heater and heated to a relatively hot temperature and then supplied to said supplemental burner to control the combustion there at, and upwardly through said combustion grate to evaporate some of the moisture from said wet fuel and facilitate combustion of such fuel, the method of heating said gas supplied to said combustion grate to a greater degree without the need of additional 'airhcaters comprising bypassing a portion of said relatively cool gas directly to said supplemental burner whereby the unbypassed portion of said relatively cool gas passing through said air heater is heated to a greater extent thereby providing gas ofa relatively high temperature to be forced upwardly through said combustion grate.

6. A method as in claim 5 wherein the relatively cool gas bypassed to said supplemental burner is mixed with said relatively hot gas from said air heater and the mixture of the combined gases is supplied to said supplemental burner.

7. A furnace for burning wet fuel comprising a combustion grate located in the combustion zone of said furnace for carrying said fuel while being burned, a supplemental burner near said combustion zone for providing supplemental heat thereat, means for providing relatively cool air, means for supplying said relatively cool air to an air heater to provide relatively hot air, means coupled to the output said air heater for passing said relatively hot air upwardly through said combustion grate to evaporate some of the moisture from said wet fuel and facilitate combustion of such fuel, and means for bypassing a portion of said air supplied to saidair heater directly to said supplemental burner to control combustion thereat whereby the unbypassed portion of said relatively cool air passing through said air heater is heated to a greater extent thereby providing air of a higher temperature to evaporate moisture from said wet fuel and facilitate combustion of same.

8. A furnace as in claim 7 wherein said air heater is located in the path of the flue gases of said furnace and is arranged to transfer heat from said flue gases to said relatively cool air.

9. A furnace as in claim 7 further comprising means for mixing a portion of said relatively hot air and said relatively cool air and for feeding the thus mixed air to said supplemental burner.

10. A furnace as in claim 7 further comprising means for cooling said combustion grate.

11. In a furnace burning wet fuel and having a supplemental burner to provide additional heat in the combustion zone thereof, a combustion grate for carrying said wet fuel to be burned, means for supplying relatively cool gas, an air heater for heating said relatively cool gas to a relatively high temperature to provide a relatively hot gas, means for coupling said relatively hot gas upwardly through said combustion grate to evaporate some of the moisture from said wet fuel and facilitate combustion of same, and means for supplying said relatively hot gas to said supplemental burner to control the combustion thereat, the improvement for heating said gas supplied to said combustion grate comprising means for bypassing said means for supplying said relatively hot gas to said supplemental bumer whereby the unbypassed portion of said relatively cooled gas is heated to a greater extent.

12. A furnace as in claim 11 further comprising means for mixing said relatively cool gas and said relatively warm gas and for supplying the resultant mixture of gases to said supplemental burner. 

1. A method of burning in a furnace wet fuel which is placed on a combustion grate located in the combustion zone of said furnace, said method comprising: providing supplemental heat at said combustion zone by operating a supplemental burner being fed with air which controls the combustion at said supplemental burner; providing relatively cool air and supplying same to an air heater to produce relatively hot air; passing at least a portion of said relatively hot air upwardly through said combustion grate to evaporate some of the moisture from said wet fuel and facilitate combustion of such fuel; and bypassing a portion of said relatively cool air supplied to said air heater directly to said supplemental burner to control combustion thereof whereby the unbypassed portion of said relatively cool air passing through said air heater is heated to a greater extent thereby providing air of a higher temperature to evaporate moisture from said wet fuel and facilitate combustion of same.
 2. A method in accordance with claim 1 wherein said air heater is an air heat exchanger and wherein said relatively hot air is produced by passing flue gas of said furnace through said air heater.
 3. A method in accordance with claim 1 wherein a portion of said relatively hot air is also supplied to said supplemental burner.
 4. A method in accordance with claim 1 and further comprising the step of cooling said combustion grate.
 5. In a method of burning wet fuel in a furnace having a supplemental burner to provide additional heat in the combustion zone thereof, wherein said wet fuel is positioned on a combustion grate in said combustion zone while relatively cool gas is passed through an air heater and heated to a relatively hot temperature and then supplied to said supplemental burner to control the combustion there at, and upwardly through said combustion grate to evaporate some of the moisture from said wet fuel and facilitate combustion of such fuel, the method of heating said gas supplied to said combustion grate to a greater degree without the need of additional air heaters comprising bypassing a portion of said relatively cool gas directly to said supplemental burner whereby the unbypassed portion of said relatively cool gas passing through said air heater is heated to a greater extent thereby providing gas of a relatively high temperature to be forced upwardly through said combustion grate.
 6. A method as in claim 5 wherein the relatively cool gas bypassed to said supplemental burner is mixed with said relatively hot gas from said air heater and the mixture of the combined gases is supplied to said supplemental burner.
 7. A furnace for burning wet fuel comprising a combustion grate located in the combustion zone of said furnace for carrying said fuel while being burned, a supplemental burner near said combustion zone for providing supplemental heat thereat, means for providing relatively cool air, means for supplying said relatively cool air to an air heater to provide relatively hot air, means coupled to the output said air heater for passing said relatively hot air upwardly through said combustion grate to evaporate some of the moisture from said wet fuel and facilitate combustion of such fuel, and means for bypassing a portion of said air supplied to said air heater directly to said supplemental burner to control combustion thereat whereby the unbypassed portion of said relatively cool air passing through said air heater is heated to a greater extent thereby providing air of a higher temperature to evaporate moisture from said wet fuel and facilitate combustion of same.
 8. A furnace as in claim 7 wherein said air heater is located in the path of the flue gases of said furnace and is arranged to transfer heat from said flue gases to said relatively cool air.
 9. A furnace as in claim 7 further comprising means for mixing a portion of said relatively hot air and said relatively cool air and for feeding the thus mixed air to said supplemental burner.
 10. A furnace as in claim 7 further comprising means for cooling said combustion grate.
 11. In a furnace burning wet fuel and having a supplemental burner to provide additional heat in the combustion zone thereof, a combustion grate for carrying said wet fuel to be burned, means for supplying relatively cool gas, an air heater for heating said relatively cool gas to a relatively high temperature to provide a relatively hot gas, means for coupling said relatively hot gas upwardly through said combustion grate to evaporate some of the moisture from said wet fuel and facilitate combustion of same, and means for supplying said relatively hot gas to said supplemental burner to control the combustion thereat, the improvement for heating said gas supplied to said combustion grate comprising means for bypassing said means for supplying said relatively hot gas to said supplemental burner whereby the unbypassed portion of said relatively cooled gas is heated to a greater extent.
 12. A furnace as in claim 11 further comprising means for mixing said relatively cool gas and said relatively warm gas and for supplying the resultant mixture of gases to said supplemental burner. 